The present invention relates to an electronic device for use in connection with a musical instrument, and more particularly to an electronic device for sustaining the vibration of a string of a stringed musical instrument.
It has long been known that an amplifier could be coupled to a stringed musical instrument to amplify the sound produced by the vibration of the strings of the instrument. Probably the most popular example of such an electrically amplified stringed musical instruments is an electric guitar. An electric guitar typically includes a plurality of strings that extend between the head of the guitar and the body of the guitar, with a fretted neck interposed between the head and body of the guitar.
In an electric guitar, one or more magnetic pickups are placed on the body of the guitar in magnetic proximity to the strings of the guitar. The magnetic pickups are responsive to the change in magnetic flux caused by the vibration of the strings. This magnetic energy picked up by the pickup is then transmitted to a separate amplifier and speaker.
It has long been known that a pickup and external amplifier arrangement on an electric guitar can not only adjust the volume of the sound produced by the guitar, but can also be used by the musician to alter the nature of the sound produced by the guitar. One means for altering this sound is to introduce vibrational feedback into the system to prolong the vibration of the strings of the guitar.
An early method for producing such sustained vibration was for the musician to move the musical instrument in close proximity to the speaker of the amplifier through which the guitar was being amplified. In such a situation, the acoustic energy caused by the sound waves emanating from the speaker of the amplifier would establish a sympathetic vibration of the strings. The vibration of the strings induced by the speaker would then be translated into magnetic flux energy picked up by the pickup means. This magnetic flux energy would then be transmitted through the external amplifier, through the separate amplifier, and would be transformed into sound energy through the speaker of the amplifier. Typically, this situation would result in a "feedback" loop which sustained the vibration of the strings of a musical instrument, and hence the duration of the sound produced by the plucking of the string.
One difficulty however with this method of introducing feedback is that it is often difficult to control the amount and type of feedback produced. Hence, it is difficult to control the sound produced through the use of this feedback system. Several devices have been invented, to overcome the problems discussed with the above method of sustaining string vibration.
A typical, prior art sustain device 8 is shown in FIG. 1 as including a magnetic pickup 10, a magnetic driver 12, and an amplifier 14 interposed in a circuit between the pickup 10 and driver 12. The pickup is typically comprised of one or more pickup coils, such as pickup coil 11. The driver 12 is typically comprised of one or more of the driver coils, such as driver coil 13.
The sustain system 8 may be used to sustain the vibration of a single string, such as string 16, or a plurality of strings, such as the 4, 6, or 12 strings typically found on an electric guitar. The sustain system is usually disposed on a counter-sunk portion of the upper surface of the body of the electric guitar, so that the pickup 10 and driver 12 are in magnetic proximity to the string 16 of the instrument.
The pickup 10 and driver 12 are constructed generally similarly. Both the pickup 10 and driver 12 are constructed of a number of turns of a conductor means, such as a wire 18, 20 which is wound around a magnetic core 22, 24, respectively. The cores 22, 24 are generally either a permanent magnet, or a ferrous material in contact with a permanent magnet, to provide a permanent magnetic flux through the center of the respective pickup coil 11 and driver coil 13.
For the purposes of this discussion as to the manner in which such a sustain system works, the pickup coil 11 and driver coil 13 are modeled as ideal inductors, L.sub.P and L.sub.D, respectively, having .sub.P, and N.sub.D, respectively, turns of wire in series with a resistive element, such as resistor R.sub.P 26 and resistor R.sub.D 28, respectively. The amplifier 14 is modeled as having infinite input impedance, zero output impedance, and a voltage gain of A. The string 16 is assumed to be under tension, free to vibrate, and secured at both ends.
A condition exists in all prior sustain systems using a magnetic pickup and driver in conjunction with an amplifier to sustain string vibration. When the gain of the amplifier 14 is of a sufficiently high level to achieve sustain of the string 16, a portion of the driver's 12 magnetic field F is present at the pickup 10. This magnetic field induces the pickup 10 to create a voltage. The pickup voltage is amplified and regenerated by the driver 16, which then is picked up by the pickup 10, to induce the pickup 10 to create a greater voltage.
When the amplifier gain is increased to the point wherein the magnetic loop gain is greater than or equal to unity, and the loop's phase angle is zero degrees, 360 degrees, 720 degrees, or some whole multiple of 360 degrees, the classical nyquist condition will be met, and the system will oscillate. Since the frequency of oscillation is generally determined by the self-resonant frequency of the pickup, the driver, and other phase and amplitude characteristics of the amplifier, the oscillation frequency has no musical relationship to the string vibration frequency. Oscillation is therefore undesirable.
A second problem associated with direct magnetic feedback between the driver and pickup is the contamination of the pickup signal with noise and distortion produced by the amplifier means. The presence of amplifier noise and distortion in the pickup signal produces an unnatural tone when the pickup is used in conjunction with a loudspeaker to monitor the tone produced by the vibrating string.
One common solution to the direct magnetic feedback problem is to decrease amplifier gain. However, this decrease in amplifier gain also reduces the ability of the system to pick up and sustain slight string vibrations. Additionally, the amount of time required for the system to reach a steady state sustain condition (where the maximum string vibration amplitude is limited by the maximum dynamic range of the system) is lengthened.
A second, prior art solution to the problems of direct magnetic feedback is to spatially separate the pickup and driver by a greater distance. One example of a device which reduces direct magnetic feedback by such a spatial separation is the SUSTAINIAC Model B sustain system, manufactured by Maniac Music, Inc. of Indianapolis Ind., which is described in the applicants' U.S. patent application Ser. No. 06/937,871, filed on Dec. 4, 1986.
In the SUSTAINIAC sustain device, the magnetic driver is a magnetic vibrational transducer which attaches to the head, stock or body of the musical instrument to provide an acoustic vibrational feedback to the string through the string supports. Although this system performs its function well, room for improvement exists. Particularly, room for improvement exists in the area of providing a more predictable phase relationship between the transducer drive current and the string vibration, as the SUSTAINIAC sustain system transducer must act on the string through the complex acoustic time delays and phase anomalies of the musical instrument's body resonance.
Another variation on this second solution is to place the pickup and driver at opposite ends of the strings. One difficulty with this method however is that it precludes the use of frets on a musical instrument. Thus, although this second method would adapt well to a piano, it adapts poorly to a guitar.
A third method of overcoming direct magnetic feedback is to eliminate one or both of the magnetic components. For example, the magnetic pickup may be replaced with a piezoelectric device, or a strain gauge which can sense string vibration while being insensitive to the driver's magnetic field.
A fourth method of overcoming the problem of direct magnetic feedback is to provide the pickup and driver with a very small air gap between the magnetic poles. The commercially available E-bow sustain system, manufactured by Gregory A. Heet of Los Angeles, Calif., and described in U.S. Pat. No. 4,075,921, embodies this type of approach. One difficulty with this approach is that the strings must be in very close proximity to the pickup and driver, and the string vibrational excursion must be minimized to avoid direct contact between the strings and the pickup and driver.
A fifth, prior art method for overcoming the problems caused by direct magnetic feedback is to provide the pickup with a humbucking apparatus to cancel the effects of uniform external magnetic fields. Such a humbucking apparatus is described by Cohen in U.S. Pat. No. 3,742,113. Cohen describes the humbucking apparatus as a "differential pickup of the type well known in the state of the art" constituted by two coils wherein "both coils respond to magnetic fields identically." One difficulty with such an approach however, is that the humbucking pickup does not provide optimum rejection of the non-uniform magnetic field generated by the driver due to the balanced design of the pickup. As will be appreciated, the driver's magnetic field is non-uniform in close proximity to the driver due to the inverse square law of magnetic field intensity. This law provides that as distance from the driver is increased, the magnetic field becomes more uniform. It will be noticed that Cohen provides a shield, consisting of layers of high and low permeability material around the perimeter of the humbucking pickup, to lessen the effects of direct magnetic feedback. The perimeter shield does not affect the magnetic balance of the humbucking pickup due to the shield's symmetry.
A variation of this fifth method for overcoming the problems of direct magnetic feedback is to provide the driver with a humbucking apparatus to allow far field cancellation of the driver's generated magnetic field. This is obvious since the driver is the electrical "dual" of the humbucking pickup described in U.S. Pat. No. 3,742,113. One problem with this approach, however is that the humbucking driver does not provide an optimally cancelled magnetic field in the proximity of the pickup, due to the balanced design of a humbucking driver.
A sixth prior art method for overcoming the problems caused by direct magnetic feedback is to provide a magnetic shield to encase the pickup. Such a shield is described in Holland's U.S. Pat. No. 4,236,433. One difficulty with this method, however, is that it encases a portion of the string and the encased string portion may not be plucked or struck.
A seventh prior art method for overcoming the problems caused by direct magnetic feedback is to provide a device wherein the pickup is located between identical drivers wired electrically out of phase. Such a device is shown in Cohen's U.S. Pat. No. 3,742,113. One difficulty with this device, however, is that it requires the drivers to be placed in "shields of magnetic ingot iron" to minimize direct magnetic feedback. A second difficulty with this device is that the driver cores must be "provided with a concave figure to focus or concentrate the flux generated on a string."
Although the above described attempts to solve the problem of direct magnetic feedback all perform their intended function, to one extent or another, room for improvement still exists.
Thus, it is one object of the present invention to provide a sustain device which maximizes the ability to sustain the vibration of a string, while minimizing the effects of direct magnetic feedback associated therewith.